1. Field of the Invention
This invention relates to an improvement of a surface temperature sensor head. A laser diode (LD) is one of the most important key devices of optical communication systems.
This invention claims the priority of Japanese Patent Application No.11-344569 (344569/1999) filed Dec. 3, 1999 which is incorporated herein by reference.
Since high density current flows in a tiny LD chip, the current produces large heat in the laser diode. The large heat raises the temperature of the LD chip. The high temperature deteriorates the properties and decreases the reliability of the LD. Further, the large heat changes the oscillation frequency of the LD or shortens the lifetime of the LD. Thermal analysis on the LDs has been energetically carried out for enhancing the property and the efficiency of LDs.
The multiwavelength transmission technology attracts attention. Application of LDs to the light source of the multiwavelength transmission requires more rigorous temperature control of the LDs, because the change of the temperature varies the oscillation frequency of the LDs. The change of the oscillation frequency also varies the performance of wavelength division multiplexers (WDMs) and the sensitivity of photodetecting devices. The temperature of the LD should be controlled for prohibiting the oscillation frequency from changing. Enhancement of the importance of the temperature control requires further progress of thermal analysis technology for measuring the temperature distribution in a tiny LD chip.
General semiconductor devices, for example, LSIs, tend to pursue the higher speed and larger data capacity. However, increasing heat generation prevents the semiconductor devices from accomplishing the higher speed and higher data capacity. A further development of the LSIs requires the progress of the thermal analysis technology.
Inspection and examination of LSIs request the technology which enables the operators to carry out electric measurements and the thermal measurements simultaneously at a high speed.
The present invention aims at answering the requests for improved thermal analysis technology. The present invention proposes a surface temperature sensor head available for the thermal analysis or inspection of the LDs, LSIs or other electronic devices. This invention will serve a sensor head of a non-destructive temperature probe for seeking temperature distribution by measuring temperature. at a plurality of small spots of an object without breaking the object.
2. Description of Related Art
The thermal analysis technology investigates spatial temperature distribution of an object by various means. The measuring of temperature requires temperature sensors. There are various kinds of temperature sensors, for example, thermocouples, thermistors, radiation thermometers and so on. The temperature sensors measure temperature on different bases.
The thermocouple is a sensor which makes use of the voltage thermally induced at the junction which is proportional to the temperature difference between two joints of two different metal wires. Since the thermocouple utilizes thermoelectromotive force, the temperature of an object is measured by fixing a tip (unction of two metals) of the thermocouple tightly to a spot of the object with a paste or silver solder of good thermal conductivity. Since the tip.should be tightly in contact with the object, the paste or solder fixes the tip on a surface of the object. The paste fixation forbids the thermocouple to move the tip. It is difficult for the thermocouple to measure temperature at a plurality of spots on an object because of the difficulty of moving the tip. The modes of measurement are restricted by the fact that the tip should be tightly touched to the object.
Some other sensors make use of the change of electric resistance depending upon temperature. They are a thermistor thermometer and a platinum thermometer resistor. The thermistor should also be fixed to an object by paste or solder for maintaining a tight contact between the sensor and object. The difficulty of moving a sensing part accompanies the thermistors.
A micro-radiation thermometer is a sensor which measures temperature at many points of an object and displays the temperature distribution on an image. Radiation power from a heated object is determined by Rayleigh-Jeans"" Law. The peak wavelength of the radiation is proportional to an inverse of the absolute temperature of the object. The whole radiation per a unit area is in proportion to a quadruple of the absolute temperature by Stefan-Boltzmann""s Law. The radiation thermometer measures the temperature or temperature distribution on a surface by the wavelength distribution and the total power of infrared radiation from the surface of the object. Unlike the thermocouples or thermistors, the radiation thermometer is a non-contact type sensor. The non-contactness enables the radiation thermometer to measure the temperature of an object from a remote spot. The radiation thermometer can obtain spatial temperature distribution by changing the directions for measuring the radiation on the object.
The purpose of the present invention is to obtain exact temperature distribution on a small device, for example, an LD, an LSI or so. Prior art which aimed at a similar purpose should be described.
{circle around (1)} Japanese Patent Laying Open No.4-191626 (191626/""92) xe2x80x9cSurface temperature measuring sensorxe2x80x9d tried to measure the exact temperature of an object by making use of a sheathed thermocouple and a copper disc. The tip of the sheathed thermocouple is fixed in the copper disc. The sensor measures the temperature of an object by boring a hole on the object and inserting the copper disc having the sheathed thermocouple into the hole.
The object is heat exchangers, steam turbines, reaction furnaces and so on. The reason why the thermocouple is sheathed by an insulator is that the thermocouple must be insulated from the metal objects. The reason why the copper disc is fixed to the tip of the thermocouple is that the temperature of the object can be measured exactly and speedy by maintaining a tight and stable contact with the object. It takes only a short time. for the object to attain thermal equilibrium with the copper disc having a high thermal conductivity and large heat capacitance. The sensor measures the temperature of the copper disc which keeps the thermal equilibrium with the object.
If the tip of the thermocouple is bluntly pushed to the object without the copper disc, unstable air gaps existing between the tip and object suppress heat conduction, inhibit thermal equilibrium and forbid the sensor to measure the exact temperature of the object. The copper disc helps the sensor of {circle around (1)} to shield noise and measure the exact temperature of the object.
In the examination of LSIs, the properties should be tested for the LSIs in a regularly driven and thermally controlled state. LSI examinations often use a xe2x80x9cprobe-card typexe2x80x9d prober having plenty of probing needles which can come into simultaneous contact with all the electrode pads on the object LSI. The examination apparatus measures the properties and characters of the LSIs by bringing all the needles of the probe-card type prober into the counterpart electrode pads of the LSI in the thermally controlled state.
The temperature control requires a couple of a heater and a temperature sensor. In general, a thermocouple is fitted on a susceptor for sustaining a wafer as a substrate for producing LSIs for protecting the wafer from contamination. The LSI testing apparatus measures the properties and characteristics by monitoring the temperature of an object LSI by the thermocouple, maintaining the LSI at a predetermined temperature by the heater, supplying the object LSI with driving currents, source and ground voltages and input signals and monitoring output currents, output voltages and output signals.
{circle around (2)} Japanese Patent Laying Open No.4-359445 (359445/""92), xe2x80x9cHeat testing probing apparatusxe2x80x9d, suggested an improvement of the wafer heat testing probe which measures electric properties by maintaining an object wafer at a predetermined temperature, bringing the needles of the probe into contact with the electrodes of the object wafer and examining currents, voltages and signals. Even if the temperature distribution is once exactly built in the object wafer, a contact of the probe needles to the wafer perturbs the once-built temperature distribution. Even a touch of a very thin needle induces extra heat conduction which disturbs the temperature of the wafer.
This improvement contrives to reduce the temperature fluctuation induced by the touch of the probe needles by maintaining the probe at the same temperature as the wafer. For the purpose, this improvement provides the probe with a Peltier device and a temperature sensor. The Peltier device can heat or cool the probe by alternating the directions of current flow. The temperature sensor is a thermocouple. The tip of the thermocouple is buried into the probe for sensing exact temperature of the probe. The reason why the tip is buried into the probe is that exact temperature can be measured by reducing the heat resistance between the tip and the probe.
{circle around (3)} Japanese Patent Laying Open No.7-74218, xe2x80x9cMethod for testing ICs and probe card for testingxe2x80x9d, proposed an improvement of a probe card for an IC wafer test. The IC wafer test is a comprehensive testing method which examines the properties and characteristics of individual IC chips with a probe card chip by chip on an object wafer which has not been cut into individual chips yet. Although the purpose is the examination of individual IC chips, the method is called a xe2x80x9cIC wafer testxe2x80x9d instead of xe2x80x9cIC chip testxe2x80x9d, since the examination is done before the wafer is scribed into separated chips.
The probe card has a lot of probing needles projecting downward from the bottom. The probing needles are used for testing electrical properties of the ICs by touching the electrode pads of the ICs. In addition to the conventional electrical property testing needles, {circle around (3)} proposed to provide the probing card with thermally testing probing needles for testing the temperature distribution of the object ICs. This improvement prepared four temperature monitoring needles per an IC chip for touching corners of the chip. The temperature monitoring needles enable the probe card to measure the temperature distribution on the surface of the IC chips easily for a short time.
The wafer having many IC chips is heated by a heater furnished in the stage. The temperature is monitored by a thermocouple in the stage. The temperature of the stage is controlled by the heater and the stage-thermocouple. However, the temperature of the wafer is not necessarily equal to the temperature of the stage. Sometimes actual temperatures of the wafer or the ICs deviate from the designation of the stage-thermocouple. {circle around (3)} suggested to add extra probe needles to the probe card for monitoring temperatures of the ICs. The temperature monitoring needles are also thermocouples.
{circle around (4)} Japanese Patent Laying Open No.11-125566, xe2x80x9cSurface temperature measuring sensor and temperature measuring probexe2x80x9d, proposed a surface temperature measuring sensor head having a low thermal conductive tube, a thermocouple piercing the tube and a high thermal conductive metal half-sphere holding the tip of the thermocouple. This application also proposed a temperature measuring probe for measuring surface temperature of an object by touching the half-spherical metal of the surface temperature sensor head to the object.
The surface temperature sensor head of {circle around (4)} is now explained by referring to FIG. 2. In the figure, the sensor head is denoted by numeral 3. A thermocouple 1 pierces through a tube 2 of low thermal conductivity. The tip of the thermocouple 1 is held by a hemispherical metal contacter 19 of high thermal conductivity. The hemispherical contacter 19 has a central hole 20. The tip of the thermocouple 1 is inserted and fixed in the central hole 20. The contacter 19 and the tip are unified. The hemispherical metal contacter 19 is made of a material having thermal conductivity higher than 100 W/mK. The contacter 19 is made of, e.g., copper. The contacter 19 has a hemispherical top and a flat bottom. The flat bottom can be brought into contact to a surface of an object. The flat bottom has a small diameter of 0.5 mm to 1 mm for detecting subtle spatial change of temperature with high resolution. The contacter 19 is a very tiny metal hemisphere.
The tube 2 pushes the hemispherical contacter 19 down via a heat insulator 18. The probe head of {circle around (4)} consists of the tube 2. the thermocouple 1, the insulator 18 and the hemispherical contacter 19.
The metal hemisphere is attached to the tip of the thermocouple for equalizing the temperature of the tip to the temperature of the object through thermal equilibrium. For the purpose, the hemisphere metal should have thermal conductivity higher than 100 W/mK. High thermal conduction is a requisite for the contacter.
The metallic contacter 19 is brought into contact to the surface of the object temporarily by the pressure from the tube. Unlike prior art {circle around (1)} and {circle around (2)}, {circle around (4)} does not fix the thermocouple to the object, e.g., with a resin. Without permanent attachment, the function of the metal hemisphere 19 rapidly brings the tip into the thermal equilibrium with the object. Since the tip is not fixed to the object, the surface temperature measuring head (probe) can be freely moved on the surface. Freedom of movement enables the sensor head to measure temperatures at lots of points for seeking temperature distribution on the surface.
The spatial resolution is restricted by the size of the hemisphere metal 19. Enhancement of spatial resolution requests a smaller metal hemisphere contacter 19. Reliability of measurement requires a bigger metal hemisphere for suppressing noise by establishing stable equilibrium. The use of the low thermal conduction tube 2 aims at preventing the header from perturbing the temperature of the object by the heat conduction through the tube to the object.
There are some proposals for seeking spatial temperature distribution on an object. However, drawbacks plague these proposals. Problems are explained about the temperature measuring method except the thermocouples.
The radiation thermometer is an apparatus of catching infrared rays radiated from a sample (object) via a microscope by a CCD (Charge Coupled Device) array and displaying the temperature distribution on a monitoring TV. When the sample is a composition of metal and glass, the components have different radiation rates and infrared light scattered by the metal surface induces external noise. The difference of radiation rates and the induced noise disturb the operation of the radiation thermometer. Carbon should be painted overall on the sample for suppressing the radiation difference and the reflection noise. The carbon coating cannot be eliminated. The way by the radiation thermometer is not a non-destructive measurement for the compounds of metal and glass. This is one weak point.
Another drawback appears in the case of samples having steps on the surface. When the sample is a ceramic packaged IC, a top of an IC is higher than the bottom of the package. The difference of the heights prohibits the microscope from tuning the focus simultaneously both to the IC top and to the package bottom. Defocus of the microscope impairs spatial resolution of the temperature distribution.
The chip thermistor or the platinum temperature measuring resistor (resistor sensors) has other drawbacks. These devices have a chip for sensing temperature and wires connecting with the chip. Owing to the sensing chips, these thermometers"" are inapt for measuring temperature at narrow regions.
The sensor chip is fixed to a sample by brazing the chip to an object via a submount with wiring patterns. Because of the brazing, non-destructive measurement is difficult for the chip thermistor or the platinum temperature measuring resistor. There is large fluctuation of the resistance among the sensors. The resistor sensors are inappropriate for simultaneous measurement of a plurality of spots by several chips. The thermistor and the platinum resistor lack sturdiness. Fragility causes inconvenience of maintenance.
The measurement by thermocouples has also problems. The temperature of an object is measured by thermoelectromotive force induced between the different metals by the difference of temperatures. The sensing part is a tip of the joint of two metals. The joint of the thermocouple is usually connected to a sample with a heat conductive paste or braze. The paste or braze stains the sample. Non-destructive measurement is difficult for the thermocouple.
{circle around (1)} proposed the attachment of a copper plate to the tip of the thermocouple for improving the heat conductivity between the thermocouple tip and object. However, such a sensor is inapt for measuring temperature at different spots on an object. Besides, another problem should be pointed out for the thermocouple making use of a kind of heat sink, for example, a copper disc.
The thermocouple requires tight contact for maintaining thermal equilibrium between the tip and the sample. Attachment of a large heat sink to the tip is effective for enhancing the tightness of contact. When the heat sink itself is large, the heat sink has a large thermal resistance. The large heat sink hinders the thermocouple in measuring temperature distribution of narrow regions on the sample. Furthermore, the large heat sink deprives the sample of heat, which perturbs the thermal equilibrium. Thus, exact measurement is prohibited by the attachment of the big heat sink.
These drawbacks recommend the employment of a smaller copper plate as a heat sink of a thermocouple. A smaller copper disc (plate) would be preferable for measuring micro-temperature distribution of an object from the standpoint of enhancing the resolution. However, it is difficult to fix a small copper disc to the tip of a thermocouple. Even if a small copper disc could be once fixed to the tip of a thermocouple by some means, the copper heat sink would easily be got off the thermocouple. Otherwise, the tip of the thermocouple would be broken owing to the attached copper disc. These difficulties accompany the Cu-carrying thermocouples.
{circle around (3)} Japanese Patent Laying Open No.7-74218 proposed a temperature prober made by adopting thermocouples as a temperature sensor. Detailed explanation about the tip of the prober is not given. Nobody knows what structure {circle around (3)} proposed for a tip of the prober.
{circle around (2)} Japanese Patent Laying Open No.4-359445 used a thermocouple for controlling the temperature of an object. It would be sufficient to measure only a spot of the object. It is inappropriate for determining temperature distribution which requires temperatures at a plurality of spots.
{circle around (4)} Japanese Patent Laying Open No.11-125566 employed a hemispherical surface temperature sensor head. It is an exquisite sensor. The production of the hemisphere requires difficult processing of boring a microhole in the copper ball and cutting the Cu ball into hemispheres. In particular, it is difficult to finish a tiny copper ball of a diameter of less than 0.8 mm. On the other hand, the copper head is so big that it is difficult to investigate micro-distribution of temperature of the object having a complex surface structure. Further, it is difficult to investigate micro-distribution on the surface.
The present invention proposes a surface temperature sensing head having a tip of a thermocouple and two high heat conductive plates sandwiching the tip of the thermocouple and a braze. Namely, the sensing head of the present invention has a three layer structure containing a first high heat conductive plate, a tip and braze layer, and a second high heat conductive plate. The brazing material has the role of joining the tip and the two plates. The plates have high heat conductivity and enough heat capacitance. The good heat conduction and sufficient heat capacitance enable the-plates to arrive at thermal equilibrium with an object rapidly. High heat conduction is required for the plates in order to establish the thermal equilibrium. Poor heat capacitance of the thin tip would bring about only unstable contact with the object, if a naked tip were in contact with the object. The plates allow the tip to reach stable equilibrium with the object through the good heat conduction and heat capacitance. Higher the heat conduction of the plates and the brazing material rises, the better the response is realized and the higher the accuracy of measurement is increased.
Too large heat capacitance is undesirable for the plates. Too big plates should also be prohibited, since the size of the plates restricts spatial resolution of measurement. The high heat conductivity of the plates means here conductivity more than 100 W/mK. Another unit of heat conduction is xe2x80x9ccal/cmsecxc2x0C.xe2x80x9d. The two units are coupled by a relation of 1 cal/cmsecxc2x0C.=420 W/mK. Materials having heat conductivity higher than 100 W/mK are, for example, copper (Cu), copper alloys, molybdenum (Mo), magnesium (Mg), silver (Ag), aluminum (Al), aluminum alloys, gold (Au) and so on. One material should be selected from the candidates for making the plates.
The sensor head of the present invention resembles the prior art {circle around (4)}. However, {circle around (4)} is a head made by boring a hole in a metallic hemisphere, piercing a tip of a thermocouple into the hole and fixing the tip in the hole. In stead of the hemisphere and hole, this invention employs one or two thin plates for sandwiching the tip between the two plates. Thin plates can easily be cut and be folded into half. Preparing simple plates is easier than making a hemisphere. The present invention dispenses with the difficult perforation of a narrow hole on the small hemisphere.
FIG. 3 is a horizontally sectioned plan view of a sensing head 3. FIG. 4 is a vertically sectioned view of the same sensing head 3. A first layer is a thin metal disc plate 22. A crossing spot 26 of a tip of a thermocouple 1 is placed upon the first metal disc 22. The crossing spot 26 is a sensing spot generating the thermoelectromotive force. The tip of the thermocouple 1 is tightly fixed to the metal disc plate 22 with a brazing material 25. A second layer 23 consists of the crossing tips 26 and the brazing material,25. A third layer is a metal disc plate 24 pressing the brazing material 25 and the sensing tips 26. The sensing head of the present invention has a three-storied structure. The lower metal plate 22 and the upper metal plate 24 sandwich the thermocouple tips 26 and the brazing 25. The metal plates should be endowed with high thermal conductivity more than 100 mW/K. The brazing material 25 unites together the upper metal plate 24, lower metal plate 22 and thermocouple tips 26. FIG. 4 shows an example having the two separated metal discs 22 and 24 which are easily produced. In short, the sensing head of the present invention includes;
the third layer 24 . . . a metal plate
the second layer 23 . . . a brazing material 25 and a crossing spot 26 of thermocouple 1
the first layer 22 . . . a metal plate, from top to bottom.
In comparison with the prior art of FIG. 2, the sensor head of the present invention dispenses with boring of a microhole 20. An advantage of this invention is the omission of the difficult boring process. Another advantage is the elimination of the difficult finishing of the small hemisphere.
The sensor head has a simple structure of piling plates for sandwiching a tip of a thermocouple which facilitates production. The production of the head dispenses with the difficult step of finishing a metal block into a hemisphere or boring a microhole through the block. An assembly of the sensor heads of the present invention can measure temperatures at a plurality of spots of a diameter of less than 0.8 mm on an object simultaneously with high accuracy. An additional contrivance allows the present invention to measure temperature at a small spot of a diameter of less than 0.3 mm. The sensor heads enable the LSI testers to examine the electronic or optical properties of LDs or LSIs with high accuracy in a short time. This invention is effective in producing a heat analysis apparatus of high resolution and high precision. The handling of the temperature prober having the sensor head is as simple as the conventional microprober instead of far higher spatial resolution than the prior microprober.